1. Field of the Invention
The present invention relates to a brake control apparatus and particularly to the art of controlling an operation force of a brake of a vehicle in a state in which the vehicle is stopped.
2. Discussion of Related Art
For example, Japanese Patent Application Publication No. 2002-154415 or Japanese Patent No. 2,749,383 discloses a brake control apparatus wherein, when a vehicle is in its stopped state, an upper limit of an operation force of a brake is made lower than an upper limit of the operation force when the vehicle is running, i.e., in its running state. In the disclosed brake control apparatus, a hydraulic brake that operates upon supplying of a hydraulic pressure to a wheel cylinder thereof is controlled such that if it is judged, based on an output signal of a wheel-speed sensor, that the vehicle is in its stopped state, an upper limit of the hydraulic pressure of the wheel cylinder is restricted, i.e., made lower than an upper limit of the hydraulic pressure when the vehicle is in its running state, so as to reduce loads applied to various elements of the brake.
In the above-described brake control apparatus, whether the vehicle is in its stopped state is judged based on a speed of a wheel of the vehicle that is represented by the output signal of the wheel-speed sensor. In this case, however, if the output signal of the wheel-speed sensor is not reliable, then a reliable judgment cannot be obtained. For example, if an electric voltage of an electric-power source that supplies an electric voltage to the wheel-speed sensor excessively lowers, then the electric voltage supplied to the sensor also lowers, and accordingly the output signal of the sensor may not be reliable. That is, although the vehicle is, in fact, in its running state, it may be erroneously judged that the vehicle is in its stopped state. In this case, if the upper limit of the hydraulic pressure of the wheel cylinder is restricted, then a braking force may be insufficiently low.
Hence, the inventors for the present application have attempted to provide a brake control apparatus that controls a hydraulic brake such that if an electric voltage of a power source that supplies an electric voltage to a wheel-speed sensor lowers to a value not greater than a pre-set value, then the wheel-speed sensor is disabled or ignored. That is, even if it may be judged based on an output signal of the wheel-speed sensor that a vehicle is in its stopped state, the judgment is ignored and an upper limit of a hydraulic pressure of a wheel cylinder of the hydraulic brake is not restricted, i.e., the hydraulic pressure is freely controlled to a value corresponding to a stroke of operation or depression of a brake pedal. In this case, when the vehicle is, in fact, not in its stopped state, a braking force of the hydraulic brake can be prevented from being insufficiently low.
However, in the above-described case, the hydraulic pressure of the wheel cylinder may be increased rapidly, and noise may be produced mainly from a hydraulic-pressure regulating valve (i.e., a pressure-increase valve). In addition, in the case where the voltage of the power source lowers just temporarily because of cranking of an engine, the power-source voltage may increase again in a considerably short time. In this case, the ignoring of the wheel-speed sensor is stopped soon, and the hydraulic pressure of the wheel cylinder is decreased rapidly, that is, noise may be produced from a hydraulic-pressure regulating valve (i.e., a pressure-decrease valve). In the case where the wheel-speed sensor is ignored because of cranking of the engine, the noise produced from the pressure-increase valve cannot be easily heard because the noise is mixed with the sounds produced by the cranking; but the noise produced from the pressure-decrease valve may be easily heard because the cranking has already ended, and accordingly one or more passengers in the vehicle feel discomfort or uneasiness. In addition, since the hydraulic pressure of the wheel cylinder is increased or decreased temporarily, a certain amount of energy is uselessly consumed.
In the case where the brake is an electric brake that includes, as a drive source thereof, an electric motor, the electric brake is free of the above-indicated problem that noise is produced from the hydraulic-pressure regulating valves. In this case, however, sounds are produced when the electric motor is operated. In addition, the electric brake suffers from the problem that a certain amount of energy is uselessly consumed.